Predawn plant water potential does not necessarily equilibrate with soil water potential under well-watered conditions

Predawn leaf water potential (Psi (w)) and xylem pressure potential (Psi (p)) are expected to be in equilibrium with the soil water potential (soil Psi (w)) around roots of well-watered plants. We surveyed 21 plant species (desert, chaparral, and coastal salt marsh species, as well as two temperate tree and two crop species) for departures from this expectation and for two potential mechanisms explaining the departures. We measured soil Psi (w), leaf Psi (w), and xylem Psi (p) in the glasshouse under well-watered conditions that eliminated soil moisture heterogeneity and ensured good soil-root hydraulic continuity. Most species failed to equilibrate fully with soil Psi (w), depending on whether leaf Psi (w) or xylem Psi (p) was used as the measure of predawn plant water potential. The contribution of nighttime transpiration to predawn disequilibrium was assessed by comparing plants with bagged canopies (enclosed overnight in plastic bags to eliminate transpiration) to plants with unbagged canopies. Nighttime transpiration significantly reduced predawn xylem Psi (p) for 16 of 21 species and the magnitude of this contribution to predawn disequilibrium was large (0.50-0.87 MPa) in four woody species: Atriplex confertifolia, Batis maritima, Larrea tridentata, and Sarcobatus vermiculatus. The contribution of nighttime transpiration to predawn disequilibrium was not more prevalent in mesic compared with xeric or desert phreatophytic compared with non-phreatophytic species. Even with bagging that eliminated nighttime transpiration, plants did not necessarily equilibrate with soil Psi (w). Plant xylem Psi (p) or leaf Psi (w) were significantly more negative than soil Psi (w) for 15 of 15 species where soil Psi (w) was measured. Predawn disequilibrium based on leaf Psi (w) was of large magnitude (0.50-2.34 MPa) for seven of those 15 species, predominately halophytes and Larrea tridentata. A portion of the discrepancy between leaf and soil Psi (w) is consistent with the putative mechanism of high concentrations of leaf apoplastic solutes as previously modeled for a halophyte, but an additional portion remains unexplained. Predawn leaf Psi (w) and xylem Psi (p) may not reflect soil Psi (w), particularly for woody plants and halophytes, even under well-watered conditions.